The present invention relates to a moving-coil type motor which is applicable to a wide range of use as a driving apparatus for effecting oscillation.
As the driving apparatus for effecting oscillation of an object, for instance, a driving apparatus for the oscillating mirror of a copying machine, a servo-motor has hitherto been generally utilized, that is, it has been a conventional way to connect this servo-motor to the oscillating mirror through some transmission means, e.g., gear, belt, cam, etc. and drive the servo-motor, thereby effecting oscillation of the oscillating mirror. However, a driving apparatus for effecting oscillation as above is defective in that, as it cannot dispense with said transmission means, the construction of it as a whole is complicated and, moreover, it requires intricate maintenance and control in order to secure accuracy and high precision of power transmission through said transmission means.
Hence, the present inventors have designed a moving-coil type motor which consists of a hollow moving coil to which electrical energy is to be applied, a magnetic circuit member made of a material having high permeability which consists of an inner portion disposed within the hollow part of said moving coil and an outer portion disposed outside the same coil, said inner and outer portions being interconnected at their rear ends, and a magnet installed on said magnetic circuit member, in which said moving coil is fixed to an arm member supported to be capable of oscillating around a shaft and is curved into a circular arc centering around said shaft, and said magnetic circuit member is also curved into a circular arc centering around the shaft similar to the moving coil. When a motor devised as above is employed for the aforesaid oscillating mirror of a copying machine, it can directly drive the oscillating mirror to effect an oscillatory movement thereof, and accordingly it is obvious that the aforesaid transmission means, e.g., gear, belt, cam, etc., become utterly unnecessary.
By the way, in order to construct the foregoing moving-coil type motor actually, the question of what shape of cross section should be chosen therefor must be settled first.
What is first conceivable is to form the cross section of the motor as a whole into a round shape. In this case, the shape of the cross section of the inner portion of the magnetic circuit member disposed in the center of the motor is to be round or annular, the shape of the cross section of the moving coil surrounding said inner portion and that of the outer portion of the magnetic circuit member surrounding the moving coil are to be annular, and the shape of the cross section of the magnet is of course to be annular. However, since it is considerably difficult to construct the respective constituent members of the motor to have such cross-sectional shapes, not only a high manufacturing technique is required therefor, but also the cost of production thereof inevitably becomes high. Especially, as regards a permanent magnet as above which has annular cross section and has axial line curved into a circular arc at that, it is practically difficult to construct it as one body by employing magnets useful in motors in general such as ferrite magnet, rare earth cobalt magnet, cast magnet, etc. or perform working thereon. For the purpose of alleviating the difficulty in constructing a permanent magnet having such shapes as above and/or working thereon, it is conceivable to take various measures such as follows: (i) application of a permanent magnet composed of a plurality of permanent magnet members as divided along the circumferential direction thereof; (ii) application of a permanent megnet composed of rubber magnet; etc. Taking of such measures is admittedly for alleviation of the difficulty in manufacturing a permanent magnet or working thereon, but on the other hand, it newly gives rise to various inconveniences such that (i') the work of disposing and fixing said plurality of permanent magnet members on the magnetic circuit member according to predetermined arrangement of positions is difficult, and (ii') in the case where said rubber magnet is employed, not only the work of fixing thereof is as difficult as the foregoing work, but also the rubber magnet per se is apt to break or crack, or as the thickness and/or the curvature of the rubber magnet depend on the adhesion strength of the adhesive employed for the work of fixing the rubber magnet, the size of the motor as a whole is also influenced thereby.
What is also conceivable is to construct a moving-coil type motor by forming the inner portion of the magnetic circuit member in the shape of a flat board having its two side edges curved into circular arc and forming the outer portion of the magnetic circuit member in the shape of a couple of parallel flat boards which are respectively connected to the rear end of the foregoing flat board and have their two side edges curved into circular arc. A motor of such a configuration has an advantage that it is easy to manufacture or perform working thereon, compared with the foregoing motor having a round cross section. On the other hand, however, it has such disadvantages that, because of the construction in which the outer portion of the magnetic circuit member does not surround the moving coil and the permanent magnet is not disposed along the whole circumference of the moving coil but is substantially fixed to the outer portion, which is in the shape of two flat boards, of the magnetic circuit member, the efficiency of the magnetic circuit member is inferior to that of the foregoing motor devised to enclose the inner constituent members with the outer constituent members, and accordingly the efficiency of the motor is inferior. Therefore, if a desired efficiency is to be obtained, the size of the motor as a whole will inevitably come to be large.